Electric sound-producing device

ABSTRACT

An electric sound producing device includes a ferromagnetic diaphragm, an electromagnet for deforming the diaphragm and an oscillator for repeatedly supplying short duration current pulses to the electromagnets. The oscillator may be driven by a multivibrator to produce intermittent sound pulses or by a sawtooth pulse generator to produce sound with an apparently cyclically varying frequency.

United States Patent 1 1 [111 3,846,792

Haigh Nov. 5, 1974 [5 ELECTRIC SOUND-PRODUCING DEVICE 3,137,846 6/1964Keeling 340/384 E 1 1 Inventor: Richard Wolliscmfl Haigh, 31233121311323 fiiiifiz'iniiiiiiiiiiii3:333:13: 318/3335 Harborough Bank,shfilsley 3,456,254 7/1969 Dougall 340/388 R Beauchamp, England3,516,088 6/1970 Allport 340/388 R [22] Filed: Nov. 5, 1971 PrimaryExammerl-iarold I. Pitts pp 196,032 Attorney, Agent, or FirmHolman &Stern [30] Foreign Application Priority Data AB TRACT N 1 9l0 gr gm iggpsmo An electric sound producing device includes a ferro- Apr. 20,1971 Great Britain ..l0310/71 magnetic diaphragm, an electromagnet fordeforming [52] U.S. Cl. 340/384 E, 340/388 the diaphragm and anoscillator for repeatedly supply- [51] Int. Cl. G08b 3/10 ing shortduration current pulses to the electromag- [58] Field of Search 340/384E, 388 nets. The oscillator may be driven by a multivibrator to produceintermittent sound pulses or by a saw-tooth [56] References Cited pulsegenerator to produce sound with an apparently UNITED STATES PATENTScycllcally s 8 q y- 3,l25,752 3/1964 Burns 340/388 R 24 Claims, 8Drawing Figures PATENTEDHUV 5 29m SHEET 2 0F 3 FIGG.

QATENTEDHM 5 1914 SHEEF 30F 8 FIG? FlG. I I

I ELECTRIC SOUND-PRODUCING DEVICE SUMMARY OF THE INVENTION Thisinvention relates to an electric sound producing device and has as anobject to provide such a device in a convenient form.

An electric sound-producing device in accordance with the inventioncomprises a resilient diaphragm of ferromagnetic material, anelectromagnet whereby said diaphragm is deformed when current issupplied to said electromagnet, and an oscillator for repeatedlysupplying current to said electromagnet in short duration pulses.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. I is a section through an example ofa device in accordance withtheinvention; I

FIG. 2 is a fragmentary section on line 22 in FIG.

DESCRIPTION OF THE PREFERRED EMBODIMENT The device shown in FIGS. 1 and2 includes a body formed of a synthetic resin material in two parts,namely a base and a cap 11. The base 10 and the cap 11 are fittedtogether with mating cylindrical surfaces so that the cap can be turnedrelative to the base 10. A plurality of abutments 12 on the base 10coact with a series of inclined ramp faces 13 in the cap to determinethe relative axial positions of the cap and base. An adhesive 14 is usedto secure the two parts together after adjustment as will be explainedhereinafter.

A diaphragm member 15 is carried by the cap 11. This diaphragm member isformed from thin steel sheet andhas a flat portion 16, a rim 17, acylindrical marginal portion 18 having an external rib l9 thereon and alip 20 around its free edge. The rib 19 is a snap fit in an internalgroove in the cap. The flat portion 16 of the diaphragm is free tovibrate in a direction perpendicular to its plane and by making itsdiameter 1 inch and its thickness 0.0] inch. a natural frequency ofapproximately 3,000 Hz can be obtained. This is a frequency to which thenormal human ear is particularly sensitive.

The base 10 has an integral spigot 21 therein extending perpendicularlyto the plane of the flat portion 16 of the diaphragm member 15. Mountedon this spigot 21 is a mild steel electromagnet core 22 oflongitudinally split tubular form. A coil 23 is mounted on this core 22.A shallow cup-shaped steel member 24 is fitted on the base part 10 withthe core 22 in contact with it around a hole in the member 24surrounding the spigot 21. The edge of the member 24 receives the lip 20of the diaphragm member 15, so that the diaphragm member 15, the core 22and the member 24 form a magnetic circuit with an air-gap between thecore 22 and the centre of the flat portion 16 of the diaphragm member15.

The drive circuit for the coil 23 has its components supported on anannular insulation member 25 itself carried by wires 26 extendingthrough holes in the member 24 and the base 10 and connected to powerinput terminals 27. One of the wires 26 is insulated to prevent theterminals 27 being short-circuited by the member 24.

As shown in FIG. 4, which illustrates a very simple circuit for thedevice, the coil 23 has two windings 28 and 29. The winding 28 has oneend connected to a positive input terminal 27 and its other end isconnected to the collector of an n-p-n transistor 30. This transistor 30has its emitter connected to the other terminal 27. A resistor R isconnected between the collector of the transistor 30 and the anode of adiode 31 which has its cathode connected to the positive terminal 27.

The other winding 29 is connected at one end to the base of thetransistor 30 and at the other end to one electrode of a capacitor C andone end of a resistor R The other electrode of the capacitor C isconnected to the negative terminal 27 and the other end of the resistorR is connected to the positive terminal 27.

The circuit described forms an oscillator which operates to produce atrainof pulses in the coil 23. When the circuit is first connected tothe battery the capacitor C charges up through the resistor R untilcurrent starts to flow to the base of the transistor 30. The resultingcollector current through the winding 28 induces a voltage in thewinding 29 which drives the transistor 30 into a fully conductingcondition. The current in winding 28 increases until the electro-magnetis saturated when the induced voltage falls and the transistor becomesnonconductive. The current in winding 28 then flows through the resistorR and the diode 31 and decays. Meanwhile, the capacitor C has beenpartially discharged and this then recharges to initiate another pulse.The pulse repetition frequency can be varied by varying the values ofthe resistor R and the capacitor C, and this frequency is chosen so asto be substantially less than the natural frequency of the diaphragm.For efficient operation the resistor R is chosen so that the pulselength is approximately equal to half the natural period of thediaphragm.

The device thus produces a train of pulses of high frequency sound. Thequality of the sound can be varied by a suitable choice of the naturalfrequency of the diaghragm, and the pulse repetition rate.

The device canbe inexpensively produced and can produce a relativelyloud noise for a small power consumption. It has the advantage overconventional electromechanical devices incorporating contact breakersthat there are no wearing parts and dirt and corrosion will notsignificantly affect the operation of the device.

to vary somewhat with variations of temperature owing to possiblevariations in the incidental resistance of the base-emitter circuit ofthe transistor 30. The inclusion of the resistor R stabilizes thisresistance and hence the pulse length. Secondly the positions of theresistor R and the diode 31 are interchanged and a capacitor C isconnected between the cathode of the diode 31 and the negative terminal27. This capacitor C reduces the level of electrical interferenceinjected back into the supply by operation of the device and also givesa measure of protection to the transistor 30 from high peak transientvoltages in the supply which may result from the operation of otherequipment, from the same supply.

Finally a diode 32 is introduced between the positive terminal 27 andthe resistor R to protect the transistor from damage caused by reversevoltages resulting from inadvertently connecting the terminals 27 to theincorrect terminals of the battery or other supply.

As a further possibility the device may be arranged to operateintermittently either by connecting the terminals 27 to the supply via athermal-type flasher unit or as shown in FIG. 6. In this case thecircuit 33 which corresponds to the circuit of FIG. with two exceptionsoperates as before to supply pulses to the winding 28. The capacitor Cis, however, shown in an alternative position between the cathode of thediode 32 and the negative terminal 27. In this position it stillprovides suppression of electrical interference, but safeguards thetransistor 30 against high peak transient voltages to a lesser extent.The other modification is the disconnection of the winding 29 from thecathode of the diode 32 and its connection instead to the outputterminal 35 of a multivibrator 34. This multivibrator is of conventionalform including two n-p-n transistors 36, 37 with their collectorsconnected via resistors 38, 39 respectively to the cathode of diode 32and their emitters connected to the negative terminal 27. The bases ofthe transistors are connected via resistors 40, 41 respectively to thecathode of the diode 32. A capacitor 42 connects the base of thetransistor 37 to the collector ofthc transistor 36. A capacitor 43 isconnected between the base of the transistor 36 and the anode of a diode44 having its cathode connected to the collector of the transistor 37. Aresistor 45 is connected between the anode of the diode 44 and thecathode of the diode 32. The output terminal 35 is connected to thecollector of the transistor 37.

It will be appreciated that the multivibrator produces an intermittentoutput. The circuit 33 will operate only when an output is received fromcircuit 34, i.e. during these periods when the transistor 37 isnon-conductive.

In the alternative embodiment of the invention shown in FIG. 3, the cap11 is replaced by a resonator 46 which fits on to the base like the cap11 and has an internal groove to receive the rib 19 on the diaphragmmember. The resonator 46 has tubular portion 47 connected at one end toan annular wall portion 48 adjacent the diaphragm member. The other endof the portion 47 has an end wall 49 which is perforated so that thetubular portion 47 is effectively and acoustically open at both ends.The length of the portion 47 is chosen so that its fundamental frequencyis approximately equal to the natural frequency of the diaphragm. Thuswhen the diaphragm is caused to vibrate a resonance condition will beestablished and the quality and volume of the sound produced will beaffected.

In the example shown the internal diameter of the tubular portion 47 isless than the diameter of the flat portion 16 of the diaphragm member.This too has an effect on the quality of sound produced by the device.In the case where the tubular portion 47 has a diameter equal to orgreater than the diameter of the diaphragm the acoustic loading on thediaphragm is low and the diaphragm/resonator combination would tend toring at a relatively low volume after initiation of vibration of thediaphragm. With a reduced diameter resonator tubular portion 47 theacoustic loading is increased so that a shorter and more intense wavetrain is produced.

Any of the three circuits of FIGS. 4 to 6 may be used in conjunctionwith the device of FIG. 3.

Mention is made above of axial adjustment of the position of the cap 11(or resonator 46) relative to the base 10 with a view to varying the airgap between the end of the core 22 and the flat portion 16 of thediaphragm member 15. Such adjustment is effected during assembly of thedevice by fitting the cap 11 and base 10 together with the adhesive 14between them and adjusting the cap 11 before the adhesive sets. Theterminals 27 are connected to a supply and the cap 11 is turned toreduce the airgap until the diaphragm is heard to strike the core, thecap is then turned in the opposite direction until such striking ceases.The adhesive is then left to set permanently fixing the cap in itsadjusted position and also sealing the body against the ingress of dirtand liquids. The holes in the base 10 through which the wires 26 arepassed would likewise be sealed.

Alternatively sealing of the body could be effected by ultrasonicwelding after adjustment of the gap.

Adjustment of the airgap to the minimum is of some importance inobtaining maximum efficiency from the device. As mentioned above thepulse length is arranged to be approximately equal to one half of thenatural vibration of the diaphragm. Thus if the diaphragm commences atrest in its normal position it will be moved towards a new equilibriumposition appropriate to the magnetic field set up by the current in thecoil. On reaching this position the diaphragm will continue to moveunder its own momentum with the magnetic attraction progressivelyincreasing.

Finally the diaphragm reaches a rest position and, ideally it is at thispoint that the pulse to the electromagnet terminates. Clearly,therefore, the total displacement of the diaphragm will increase withreduction of the air gap for the same power consumption.

In the example shown in FIG. 7 the cap 11 or resonator 46 is replaced byan exponential horn 50. Once again the narrow end of this horn is ofsmaller diameter than that of the diaphragm to place a high acousticload on the diaphragm. A high intensity sound can thus be obtained.

The circuit of FIG. 8 utilises an oscillator 33 as before, with asaw-tooth pulse generator circuit 59 including a capacitor 51 connectingthe resistor R to the negative terminal 28 and a further resistor 52connecting it to the positive terminal 27. Also connected to theresistor R is the emitter of a p-n-p transistor 53 the collector ofwhich is connected to the negative terminal via a resistor 54. The baseof the transistor 53 is connected via a resistor 55 to the negativeterminal and via a resistor 56 to the base of another -p-n-p transistor57, the collector of which is connected to the base of the transistor53. A capacitor 58 connects the base of the transistor 57 to thecollector of the transistor 53. The

emitter of the transistor 57 is connected to the positive terminal.

The cycle of operation of the circuit may be regarded as commencing withthe capacitor 51 uncharged and the transistor 53 non-conductive. Whenthe supply is connected the capacitor 51 charges up so that initiallythe voltage on the resistor R is at the negative terminal voltage butthis rises as the capacitor 51 charges, progressively increasing thepulse repetition frequency of the oscillator. When the voltage acrossthe capacitor 51 exceeds the bias applied to the base of the transistor53, the latter starts to conduct and the changing voltage at itscollector, caused by current flow through the resistor 54, is applied tothe base of the transistor 57 via the capacitor 58. This in turn causesthe bias on the base of the transistor 53 to be reduced and theregenerative action causes transistor 53 to switch fully on anddischarge the capacitor 51 extremely rapidly. The cycle then restarts.

Thus the device emits pulses of sound at the resonant frequency of thediaphragm, which ispreferably as mentioned above, in the region of 3,000Hz. The pulse repetition frequency varies cyclically from a lowfrequency rising to a high frequency and then commencing at the lowfrequency again. The total effect is a sound of which the apparentfrequency follows this pattern, although in fact all the sound emittedis at or near the aforementioned resonant frequency.

If desired, an additional transistor may be introduced between theresistor R and the capacitor 51 to allow the magnitude of the currenthandled by the timebase" circuit to be reduced. Alternatively theadditional transistor may be connected to the base of the transistor 30.In each case the additional transistor would be connected as an emitterfollower.

The horn of FIG. 7 may also be used in conjunction with any of thecircuits shown in FIGS. 4, 5 and 6.

I claim:

1. An electrical sound-producing device comprising a resilient diaphragmof ferromagnetic material having a substantially flat portion, anelectromagnet including a tubular core having one end adjacent said flatportion of the diaphragm and spaced therefrom by an air gap, the otherend thereof being mounted on a spigot, whereby said diaphragm isdeformed when current is supplied to said electromagnet, and anoscillator for repeatedly supplying current in short duration pulses tosaid electromagnet.

2. A device as claimed in claim 1 in which the oscillator producespulses of duration approximately equal to half the period of the naturalvibration of the diaphragm.

3. An electrical sound producing device comprising a resilient diaphragmof ferromagnetic material, an electromagnet whereby said diaphragm isdeformed when current is supplied to said electromagnet, saidelectromagnet including a core having one end adjacent said diaphragm, acoil comprising a pair of windings on said core and an oscillator ofwhich said core and windings form an essential part for repeatedlysupplying current to the windings in short duration pulses.

4. A device as claimed in claim 3 in which said oscillator comprisesfirst and second terminals, an n-p-n transistor with its collectorconnected via one winding of said coil to the first terminal and itsemitter connected to the second terminal, a first resistor and a diodein series circuit bridging said first winding, a capacitor connected ina circuit with the second winding of the coil connecting the secondterminal to the base of the transistor and a charging circuit for saidcapacitor including a second resistor.

5. A device as claimed'in claim 4 in which said charging circuitconsists of said second resistor connected between the end of the secondwinding which is connected to the capacitor and the first terminal.

6. A device as claimed in claim 4 in which said charging circuitcomprises a further resistor connecting the second resistor to the firstterminal, a further transistor with its collector/emitter pathconnecting the interconnection of the second and further resistors tosaid second terminal and means for applying a signal to the base of saidfurther transistor.

7. A device as claimed in claim 6 in which said further resistor andsaid further transistor form part of a multivibrator whereby thecharging circuit is energized intermittently so that the device producessound intermittently.

8. A device as claimed in claim 6 in which said further resistor andsaid further transistor form part of a sawtooth pulse generator circuitwhereby the pulse repetition frequency of the oscillator is variedcyclically.

9. A device as claimed in claim 4 further comprising a further diodewith its anode connected to the first terminal to prevent reversevoltages being applied to the first mentioned transistor.

10. A device as claimed in claim 4 in which the anode of the diode isconnected to the collector of the transistor and its cathode isconnected to the first resistor, and further comprising a furthercapacitor connecting the cathode of said diode to the emitter of thetransistor.

11. A device as claimed in claim 4 further comprising a furthercapacitor interconnecting the first and second terminals.

12. A device as claimed in claim 4 further comprising a third resistorconnecting said capacitor to the second winding.

13. An electrical sound producing device comprising a body formed in twoparts, a flexible resilient diaphragm of ferromagnetic materialsupported by one body part, an electromagnet supported by the other bodypart whereby the diaphragm is deformed when current is supplied to theelectromagnet, means for adjusting the position of said two partsrelative to one another during assembly of the device to vary the airgap between the electromagnet and the diaphragm, sealing means joiningthe body parts together in an adjusted relationship and an oscillatorcontained within said body for repeatedly supplying current to theelectromagnet in short duration pulses.

14. A device as claimed in claim 13 in which said parts have matingcylindrical surfaces and are rotatable relative to one another about thecommon axis of said surfaces during assembly, said parts havinginterengaging means for displacing the two parts axially relative to oneanother on turning of one part relative to the other.

15. A device as claimed in claim 14, in which said interengaging meanscomprises a plurality of abutments on one part and a plurality ofinclined ramps on the other part.

16. A device as claimed in claim 13, in which the part of the body whichsupports the diaphragm has an internal groove into which an external ribon a member of which the diaphragm is an integral part is a snap tit.

17. A device as claimed in claim 13, in which the part of the body whichcarries the electromagnet has an integral spigot on which there isfitted a tubular core forming part of said electromagnet.

18. A device as claimed in claim 17, in which acoil of the electromagnetis mounted on the core.

19. A device as claimed in claim 18, in which there is provided anannular insulation member surrounding the coils and supporting thecomponent parts of the oscillator.

20. A device as claimed in claim 17, in which the electromagnet furthercomprises a cup-shaped member, having a hole fitted on said spigot andengaged by the core, forming a magnetic connection between the core andthe periphery of the diaphragm, an air gap being defined between thecore and the center of the diaphragm.

21. An electrical sound-producing device comprising a body, a flexibleresilient diaphragm mounted in said body, a tubular acoustic resonatoron the body tuned to the natural frequency of the diaphragm and whoseaxis is perpendicular to the plane of the diaphragm, an electromagnet inthe body for deforming the diaphragm when current is supplied theretoand an oscillator contained in the body for repeatedly supplying currentto the electromagnet in short duration pulses of lengths approximatelyequal to half the period of the natural vibration of the diaphragm, andwherein the internal diameter of the resonator is less than the diameterof the diaphragm.

22. An electrical sound-producing device comprising a body, a diaphragmwithin the body dividing the interior of the body into two chambers, ahorn on the body opening into one chamber and of increasingcrosssectional area in a direction away from said one chamher, anelectromagnet having a pair of windings in the other chamber fordeforming the diaphragm when the current is supplied to theelectromagnet, and an oscillator circuit including said pair of windingswithin said other chamber for repeatedly supplying current to theelectromagnet in short duration pulses.

23. A device as defined in claim 22, further comprising a pulse ratecontrol circuit in said other chamber connected to said oscillator andcyclically varying the pulse repetition rate of the oscillator.

24. A device as defined in claim 23, in which said pulse rate controlcircuit is a saw-tooth pulse generator circuit whereby the pulserepetition frequency of the oscillator is caused to commence each cycleat a low frequency and rise continuously to a high frequency,

thereafter recommencing at said low frequency.

1. An electrical sound-producing device comprising a resilient diaphragmof ferromagnetic material having a substantially flat portion, anelectromagnet including a tubular core having one end adjacent said flatportion of the diaphragm and spaced therefrom by an air gap, the otherend thereof being mounted on a spigot, whereby said diaphragm isdeformed when current is supplied to said electromagnet, and anoscillator for repeatedly supplying current in short duration pulses tosaid electromagnet.
 2. A device as claimed in claim 1 in which theoscillator produces pulses of duration approximately equal to half theperiod of the natural vibration of the diaphragm.
 3. An electrical soundproducing device comprising a resilient diaphragm of ferromagneticmaterial, an electromagnet whereby said diaphragm is deformed whencurrent is supplied to said electromagnet, said electromagnet includinga core having one end adjacent said diaphragm, a coil comprising a pairof windings on said core and an oscillator of which said core andwindings form an essential part for repeatedly supplying current to thewindings in short duration pulses.
 4. A device as claimed in claim 3 inwhich said oscillator comprises first and second terminals, an n-p-ntransistor with its collector connected via one winding of said coil tothe first terminal and its emitter connected to the second terminal, afirst resistor and a diode in series circuit bridging said firstwinding, a capacitor connected in a circuit with the second winding ofthe coil connecting the second terminal to the base of the transistorand a charging circuit for said capacitor including a second resistor.5. A device as claimed in claim 4 in which said charging circuitconsists of said second resistor connected between the end of the secondwinding which is connected to the capacitor and the first terminal.
 6. Adevice as claimed in claim 4 in which said charging circuit comprises afurther resistor connecting the second resistor to the first terminal, afurther transistor with its collector/emitter path connecting theinterconnection of the second and further resistors to said secondterminal and means for applying a signal to the base of said furthertransistor.
 7. A device as claimed in claim 6 in which said furtherresistor and said further transistor form part of a multivibratorwhereby the charging circuit is energized intermittently so that thedevice produces sound intermittently.
 8. A device as claimed in claim 6in which said further resistor and said further transistor form part ofa sawtooth pulse generator circuit whereby the pulse repetitionfrequency of the oscillator is varied cyclically.
 9. A device as claimedin claim 4 further comprising a further diode with its anode connectedto the first terminal to prevent reverse voltages being applied to thefirst mentioned transistor.
 10. A device as claimed in claim 4 in whichthe anode of the diode is connected to the collector of the transistorand its cathode is connected to the first resistor, and furthercomprising a further capacitor connecting the cathode of said diode tothe emitter of the transistor.
 11. A device as claimed in claim 4further comprising a further capacitor interconnecting the first andsecond terminals.
 12. A device as claimed in claim 4 further comprisinga third resistor connecting said capacitor to the second winding.
 13. Anelectrical sound producing device comprising a body formed in two parts,a flexible resilient diaphragm of ferromagnetic material supported byone body part, an electromagnet supported by the other body part wherebythe diaphragm is deformed when current is supplied to the electromagnet,means for adjusting the position of said two parts relative to oneanother during assembly of the device to vary the air gap between theelectromagnet and the diaphragm, sealing means joining the body partstogether in an adjusted relationship and an oscillator contained withinsaid body for repeatedly supplying current to the electromagnet in shortduration pulses.
 14. A device as claimed in claim 13 in which said partshave mating cylindrical surfaces and are rotatable relative to oneanother about the common axis of said surfaces during assembly, saidparts having interengaging means for displacing the two parts axiallyrelative to one another on turning of one part relative to the other.15. A device as claimed in claim 14, in which said interengaging meanscomprises a plurality of abutments on one part and a plurality ofinclined ramps on the other part.
 16. A device as claimed in claim 13,in which the part of the body which supports the diaphragm has aninternal groove into which an external rib on a member of which thediaphragm is an integral part is a snap fit.
 17. A device as claimed inclaim 13, in which the part of the body which carries the electromagnethas an integral spigot on which there is fitted a tubular core formingpart of said electromagnet.
 18. A device as claimed in claim 17, inwhich a coil of the electromagnet is mounted on the core.
 19. A deviceas claimed in claim 18, in which there is provided an annular insulationmember surrounding the coils and supporting the component parts of theoscillator.
 20. A device as claimed in claim 17, in which theelectromagnet further comprises a cup-shaped member, having a holefitted on said spigot and engaged by the core, forming a magneticconnection between the core and the periphery of the diaphragm, an airgap being defined between the core and the center of the diaphragm. 21.An electrical sound-producing device comprising a body, a flexibleresilient diaphragm mounted in said body, a tubular acoustic resonatoron the body tuned to the natural frequency of the diaphragm and whoseaxis is perpendicular to the plane of the diaphragm, an electromagnet inthe body for deforming the diaphragm when current is supplied theretoand an oscillator contained in the body for repeatedly supplying currentto the electromagnet in short duration pulses of lengths approximatelyequal to half the period of the natural vibration of the diaphragm, andwherein the internal diameter of the resonator is less than the diameterof the diaphragm.
 22. An electrical sound-producing device comprising abody, a diaphragm within the body dividing the interior of the body intotwo chambers, a horn on the body opening into one chamber and ofincreasing cross-sectional area in a direction away from said onechamber, an electromagnet having a pair of windings in the other chamberfor deforming the diaphragm when the current is supplied to theelectromagnet, and an oscillator circuit including said pair of windingswithin said other chamber for repeatedly supplying current to theelectromagnet in short duration pulses.
 23. A device as defined in claim22, further comprising a pulse rate control circuit in said otherchamber connected to said oscillator and cyclically varying the pulserepetition rate of the oscillator.
 24. A device as defined in claim 23,in which said pulse rate control circuit is a saw-tooth pulse generatorcircuit whereby the pulse repetition frequency of the oscillator iscaused to commence each cycle at a low frequency and rise continuouslyto a high frequency, thereafter recommencing at said low frequency.